Electrophotography is one type of image-forming technique which generally includes charging the surface of a photoreceptor containing a photoconductive material by, e.g., corona discharge in the dark, exposing the charged surface to light to selectively eliminate the charge in the exposed areas and thereby obtain an electrostatic latent image, developing the latent image with a toner, and then transferring and fixing the toner to paper or the like to obtain an image. Inorganic photoconductive substances such as selenium, zinc oxide, cadmium sulfide, and silicon have hitherto been used extensively in photoreceptors for electrophotography. Although these inorganic substances have many merits, they have had various drawbacks. For example, selenium has drawbacks that it necessities difficult production conditions and that selenium is apt to crystallize with heat or mechanical impact. Zinc oxide and cadmium sulfide have problems concerning moisture resistance and mechanical strength and further have a drawback that these substances deteriorate in suitability for charge or exposure by the action of a dye added as a sensitizer, resulting in poor durability. Silicon also necessitates difficult production conditions and further necessitates use of a highly irritant gas, resulting in a high cost. Silicon is sensitive to moisture and, hence, care should be taken in handling. In addition, selenium and cadmium sulfide have a problem concerning toxicity.
Organic photoreceptors which employ various organic compounds and in which those drawbacks of inorganic photoreceptors have been mitigated are in extensive use. The organic photoreceptors include single-layer type photoreceptors in which a charge-generating agent and a charge-transporting agent have been dispersed in a binder resin and multilayer type photoreceptors in which functions have been allotted to a charge-generating layer and a charge-transporting layer. Organic photoreceptors of the function allocation type are being extensively used because there is a wide choice from which each material can be selected and because a photoreceptor having any desired performances can be relatively easily produced by selecting a suitable combination.
As the charge-generating agent, many organic pigments and dyes have been proposed and put to practical use. Examples thereof include azo compounds, bisazo compounds, trisazo compounds, tetrakisazo compounds, thiapyrylium salts, squarylium salts, azulenium salts, cyanine dyes, perylene compounds, metal-free or metal phthalocyanine compounds, polycyclic quinone compounds, thioindigo compounds, and quinacridone compounds.
Examples of the charge-transporting agent include oxadiazole compounds (see, for example, patent document 1), oxazole compounds (see, for example, patent document 2), pyrazoline compounds (see, for example, patent document 3), hydrazone compounds (see, for example, patent documents 4 to 7), diamine compounds (see, for example, patent document 8), stilbene compounds (see, for example, patent documents 9 to 11), and butadiene compounds (see, for example, patent document 12). Organic photoreceptors employing these charge-transporting agents have excellent properties, and some of these have been put to practical use. However, an organic photoreceptor which fully satisfies various properties required of photoreceptors for electrophotography has not been obtained so far. Those charge-transporting agents include ones which have satisfactory properties including sensitivity but are poor in compatibility with resins and solubility in solvents and which hence have not been put to practical use.    Patent Document 1: JP-B-34-005466    Patent Document 2: JP-A-56-123544    Patent Document 3: JP-B-52-041880    Patent Document 4: JP-B-55-042380    Patent Document 5: JP-B-61-040104    Patent Document 6: JP-B-62-035673    Patent Document 7: JP-B-63-035976    Patent Document 8: JP-B-58-032372    Patent Document 9: JP-B-63-018738    Patent Document 10: JP-B-63-019867    Patent Document 11: JP-B-3-039306    Patent Document 12: JP-A-62-030255
Among the patent documents which have been filed, some disclose the use of a p-terphenyl compound in an electrophotographic photoreceptor (see, for example, patent documents 13 and 14). Patent document 13 discloses compounds including a p-terphenyl compound. The technique disclosed in this document is intended to improve electrophotographic characteristics including durability and sensitivity by incorporating the p-terphenyl compound into the charge-generating layer of a multilayer type photoreceptor. On the other hand, the p-terphenyl compound disclosed in patent document 14 has insufficient properties including durability, although excellent in solubility.
A charge-transporting agent for use in organic photoreceptors is required not only to enable the photoreceptors to satisfy electrical properties including sensitivity but also to have chemical stability which enables the agent to withstand light, ozone, and electrical load and stability or durability which prevents the photoreceptors from decreasing in sensitivity with repetitions of use or during long-term use. Furthermore, in producing an organic photoreceptor, high and stable solubility in solvents is necessary. However, no compound has been found which has excellent solubility, is excellent in stability and durability, and is satisfactory.    Patent Document 13: JP-A-61-129648    Patent Document 14: JP-B-6-073018